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Plasmids and co-selection

The bacterial chromosome is made up of DNA and stores all the information that a bacterial cell needs to carry out its normal function. In addition to the chromosome, bacteria can have small circles of DNA called plasmids that also contains genes.

Antibiotic resistance genes were not common on plasmids before the introduction of antibiotics as medicines. Today, however, genes that provide protection against different antibiotics as well as resistance to for example heavy metals like copper and silver are commonly found on the same plasmids. One of the antibiotics (or metals) is then enough to maintain (select for) all the antibiotic resistance mechanisms since they are linked together on the plasmid. This phenomenon is called co-selection of resistance genes.

Figure 1. Plasmids are small DNA circles outside the bacterial chromosome. Several antibiotic resistance genes can be present on the same plasmid. In this example, they are called res A, res B and res C. Res A gives resistance to antibiotic A, res B to antibiotic B and so on. Adding antibiotic A (or antibiotic B or C) will select for all three resistance genes since they are on the same plasmid.

Plasmids can transfer between different bacteria

Plasmids also often have mechanisms for transfer of the whole plasmid to other bacteria. This means that a bacterium can become resistant to multiple antibiotics at once by picking up a single plasmid. They then become multidrug-resistant. Furthermore, genes that influence bacterial virulence are also frequently found on plasmids. Pathogenic bacteria carrying multidrug-resistance plasmids have increased in prevalence and now cause major problems all around the world. There are very few treatment options remaining for such infections. Learn more about the transfer of resistance genes between bacteria here: Antibiotic resistance – Transfer of resistance genes.

Co-selection is one of the reasons for why it is difficult to reverse resistance once it has been established in a bacterial population. It is simply not enough to stop using one particular antibiotic if the resistance mechanisms is linked to resistance for other antibiotics.